LASER GUIDE STAR: LASER REFERENCES

Briefly, at an altitude of about 90 km there is a layer of sodium which is about 10 km thick. This sodium has a resonant transition at about 590 nm. This wavelength is near the maximum emission of high-power dye lasers using rhodamine 6G dye. When the sodium is illuminated at this wavelength it produces an emission. This emission is what is called a laser guide star. This is used, in conjunction with adaptive optics, to correct for atmospheric turbulence at large terrestial telescopes.